The present invention pertains to electrostatic spray nozzles, and more particularly to a nozzle for emitting liquids and other flowable materials upwardly onto a target in a highly controllable and efficient fashion at a highly increased rate of material flow through the nozzle.
The application of fluids and other flowable materials onto a substrate using electrostatically operated nozzles has been heretofore proposed. One nozzle apparatus of the proposed type is found in Escallon's U.S. Pat. No. 4,749,125; the nozzle has a housing with mutually tapering sides that form a pointed dispensing end. There is a fluid duct joining a fluid reservoir to the nozzle housing interior. Fluid is introduced to the nozzle by a fluid delivery system at sufficient pressure to deliver fluid to the dispensing end of the electrostatic nozzle. As the fluid travels within the nozzle housing, it is electrostatically charged and upon reaching the emitting end forms a meniscus and subsequently erupts into a plurality of flow paths. Today there is an increasing demand for a nozzle of this character that can accomplish larger flow rates utilizing a broader spectrum of different flowable materials than ever before. Additionally, industry demands a new nozzle configuration capable of effectively emitting flowable materials upwardly for coating or covering the underside of a target.
Nozzles typical of Escallon, however, are limited in their ability to effectively spray upwardly an amount of flowable material that will meet all of these demands. One problem is "flooding". Because of the nozzle's orientation in an upwardly spraying position and a lack of hydraulic or pneumatic forces on the fluid or flowable material, the gravitational forces must be overcome in order to emit the flowable material upwardly. Depending on the viscosity and/or surface tension of the flowable material, flooding of the nozzle tip is a frequent and formidable occurrence which may be caused by a momentary loss of high voltage. At the onset of flooding, physical forces of the flowable material such as surface tension and adhesion to nozzle surfaces create a path leading the fluid down and away from the emitting edge. This fluid path cannot always be overcome with electrostatic forces. Although an upward spray still occurs, an uncontrollable percentage of flowable material begins to stream over the emitting edges of the nozzle. Eventually, the flowable material begins to misfire from the nozzle tip at locations that preclude controlled coating of the overhead target and in some instances misses the target altogether.
The overflowing or flooding of the nozzle can be corrected by shutting down the nozzle, wiping the outer portion of the nozzle emitting edge, and restarting the spray. However, since many of these lines of production are intended to be continuous operations, shut down and wiping of the nozzle is neither an economical nor an acceptable procedure.
Another complication is the "purge cycle" that is incorporated in some industrial operations. Purge, in essence, flushes the thru-put material out of the system and replaces it with another material at high volume flow rates. This flushing cycle causes a forced hydraulic flooding of the nozzle. Depending on the variety of work being processed on the line, purging may occur several times each day. The purge cycle thoroughly drenches the emitting edge of the nozzle as the materials are flushed through the nozzle. Consequently, flooding and misfiring often result when attempting to restart the system.
An alternative nozzle for dispensing flowable materials upwardly, described in U.S. Pat. No. 4,830,872 utilizes a nozzle blade having two side pieces with a space therebetween in a vertical orientation. The flowable material exits the space and is charged with a working potential of 50 to 120 kv. An electrostatic field is established between the blade end and the object to be coated. The charge has to be applied in a reliable manner taking into consideration aspects of personal safety. Hazards include sparking or arcs in the presence of potentionally flamatory solvent-borne materials, such as paint, as well as the potential for operator shock. Energy efficiency is also an important factor.
It is therefore highly desirable to provide an improved electrostatic spray nozzle.
It is also highly desirable to provide an improved electrostatic spray nozzle and method that is capable of spraying upwardly.
It is also highly desirable to provide an improved electrostatic spray nozzle and method that is capable of spraying upwardly at relativity higher flow rates.
It is also highly desirable to provide an improved electrostatic spray nozzle and method that is self-correcting and will overcome the affects of "flooding" without operator assistance.
It is also highly desirable to provide an improved electrostatic spray nozzle and method capable of avoiding the flooding problems characteristic of a more or less vertical nozzle orientation where gravitational forces affect spraying ability.
It is also highly desirable to provide an improved electrostatic spray nozzle and method capable of overcoming the adhesive forces of flowable materials and nozzle surfaces.
It is also highly desirable to provide an improved electrostatic spray nozzle and method capable of overcoming the surface tension forces of flowable materials.
It is also highly desirable to provide an improved electrostatic spray nozzle and method that are immune to the characteristics attributable to a purge cycle.
It is also highly desirable to provide an improved electrostatic spray nozzle and method that need not be shut down in an operation requiring continuous production.
It is also highly desirable to provide an improved electrostatic spray nozzle and method that operates at economically efficient and operator-safe voltage and current levels.
It is finally highly desirable to provide an improved electrostatic spray nozzle and method having all of the above-mentioned characteristics.